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    SubjectRe: Device loses barrier support (was: Fixed patch for simple barriers.)
    > * barrier support in md-raid1 deviates from the specification at 
    > Documentation/block/barrier.txt. The specification says that requests
    > submitted after the barrier request hit the media after the barrier
    > request hits the media. The reality is that the barrier request can be
    > randomly aborted and the requests submitted after it hit the media before
    > the barrier request.

    Yes the spec should be probably updated.

    But also see Linus' rant from yesterday about code vs documentation.
    When in doubt the code wins.
    >
    > * the filesystems developed hacks to work around this issue, the hacks
    > involve not submitting more requests after the barrier request,

    I suspect the reason the file systems did it this way is that
    it was a much simpler change than to rewrite the transaction
    manager for this.

    > synchronously waiting for the barrier request and eventually retrying it.
    > These hacks suppress any performance advantage barriers could bring.
    >
    > * you submit a patch that makes barriers even more often deviate from the
    > specification and you argue that the patch is correct because filesystems
    > handle this deviation.

    Sorry what counts is the code behaviour, not the specification.

    -Andi

    --
    ak@linux.intel.com


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